BY TOM FALCON, DEK
Demands to reduce the dimensions of even the most highly miniaturized semiconductor packages are placing pressure on IC vendors and packaging specialists to develop faster and more productive package-assembly processes. Ball placement techniques are being fine-tuned to address these issues.
Since the wafer-level chip-scale package (WLCSP) has become the package technology of choice for devices such as DRAMs, flash memories and small FPGAs, research has focused here to find suitable miniaturization measures. The next evolutionary step for WLCSP is to reduce the ball-grid interconnect array pitch to 0.3 mm, which allows semiconductor manufacturers to increase the number of die per wafer by more than 50%, for little extra manufacturing cost. However, continued shrinkage of pitches and ball diameters brings challenges of implementing reliable and repeatable processes for placing 0.2-mm solder balls on a 0.3-mm pitch at high yield and throughput.
Ball Placement Techniques
Over the last 10 years, several methods for the mass transfer of solder balls to wafers has been developed, as well as complementary processes for rework and single-ball placement. An accurate and repeatable stencil printing ball placement process for 0.2-mm solder balls on a 0.3-mm pitch was developed, with the goal of demonstrating greater than 99.99% placement yield at a throughput of 60UPH or more. These are the fundamental criteria for a ball-attach process supporting cost-effective mass production of 0.3-mm pitch WLCSPs.
Equipment and Process Overview
Solder-ball placement using screen printing begins with tooling the wafer in a conveyorized aluminum pallet. This pallet is transported into the flux-printing machine where the wafer is visually aligned with an emulsion-mesh fluxing screen with reference to laser-marked fiducials. Flux is then screen-printed onto all solder-bump pads. After flux printing, the wafer and pallet are transported to the ball-placement machine. Following accurate visual alignment with the metal ball-placement stencil, the wafer is brought into contact with the underside of the stencil before the ball transfer head traverses the topside of the stencil. This deposits a single solder ball into each of the stencil’s apertures (Figure 1). The alignment process ensures that the apertures coincide accurately with the fluxed solder-bump pads.
 Figure 1. Solder ball placement. |
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Screen and Stencil
The specification of the emulsion-mesh screen used for the fluxing process is derived from established formulas based on the pitches and ball diameters to be placed. Typically this will be a stainless-steel, 45